"Frankly, there is no way to make this come out happy if you comingle your government and your private emails and then put all of them on a private server as opposed to a government server.

"You're just setting in motion a whole series of things and it doesn't require anyone to be stupid or malevolent. If you set it up that way, it's going to end up in a bad place and that's the bad place we're in now."

Clinton, the presumed Democratic presidential candidate, is being investigated by the FBI over her use of a private email address and server as secretary of state — and the possibility that top secret materials were exchanged on the unsecured system.

Court documents filed Monday showed federal investigators, who are combing through Clinton's email server, have flagged more than 300 emails that passed through her email account that may have contained classified information.

Does Hayden believe Clinton's emails had a high probability of ending up in the hands of foreign intelligence services?

"I won't give you a number, but a foreign intelligence service of some merit, if they were interested in those emails, I would give them a high probability of success that they would be able to penetrate that system," he said.

As to just how much of a breach of national security Clinton's use of a private email account and server could be, Hayden said:

"The really correct answer is we don't know. We don't know the character and the content of the information that was on this private server.

"It's already been indicated that at least the intelligence community's inspector general believes some of that data was classified and a couple of messages contain information that he believes were classified top secret.

"That's not a trivial deal at all and we just have to get to the bottom of it to understand what may or may not have been out there."